There are many practical uses for systems which automatically detect the presence of an object or a person within a predetermined space; such systems are herein referred to as presence detecting systems whereas the technical literature and various patents sometimes refer to such systems as intrusion detecting systems. Presence detection systems have a wide variety of applications or uses which for the most part may be regarded as so-called "security systems" or as so-called "safety systems". A security system, such as a burglar alarm, is adapted to sense the unauthorized presence and sound an alarm when a person or other intruder is within a given zone which is to be secured against unauthorized presence or intrusion. When a presence detection system is employed as a safety system it is adapted to help safeguard a person against injury by some hazardous device; it is commonly employed with industrial machines to stop the machine or give a warning when an object or a person is in a hazardous position. In such applications the prior art presence detection systems have been unsatisfactory in several respects. For example, there has been a need for a presence detection system which is substantially immune to ambient conditions and false signals and yet which responds without fail to a genuine signal. Furthermore, because of the nature of the use of such systems there is need to provide self-checking of the system operability so that failure thereof is made known so that further reliance will not be placed on the system.
Presence detection systems have heretofore been proposed using radiant energy or light beams to define a boundary of the space or region which is to be guarded or maintained under surveillance. Such systems generally utilize one or more light sources and one or more light sensors so that interruption of the light therebetween is adapted to produce a control signal indicative of the presence of an object in the guarded region. Such a light beam system is admirably suited to machine safety systems in that it provides a well defined boundary for the guarded region. Thus the machine operator is given complete freedom of movement on the safe side of the boundary or barrier and the safety system is unaffected by movement of materials and equipment in close proximity of the boundary; however, even the slightest penetration of the boundary will cause the safety system to respond.
The prior art photoelectric presence sensing systems have used a wide variety of arrangements of light sources and photodetectors to provide a light barrier plane or "curtain". Typical of such systems is a linear array of multiple light sources and an opposed linear array of photodetectors, each of which is aligned with one of the light sources. The light sources are energized simultaneously and repetitively in a pulsing fashion and the photodetector output signals are suitably processed to determine whether any of the photodetectors are not illuminated during each pulsing interval. Such an arrangement is shown in the MacDonald U.S. Pat. No. 3,704,396. A similar arrangement is shown in the Endl U.S. Pat. No. 3,742,222. It has also been proposed to provide a light curtain by sequentially energizing sources which are disposed in a linear array; more particularly spaced columns are each provided with alternating light sources and photodetectors and the light from the source in one column is received by the opposed photodetector in the other column, which causes the adjacent source to be energized to send light to the next receiver in the first column and so on to the ends of the columns, then the cycle is repeated. In such systems there is a serious problem of crosstalk between channels, i.e. the paths between a light source and its associated photodetector. Furthermore, the sequential switching arrangement depends upon each light source and photodetector being operative in order to complete a cycle.
One of the difficulties in the photodetector type of presence sensing systems has been the effect of changes in ambient conditions, especially changes in ambient light. Several schemes have been proposed in the prior art to provide some degree of immunity of the system to such ambient light changes. For example, one such scheme discriminates between ambient light change and signal light change on the basis of the time rate of change and circuitry is provided to develop an output corresponding to the signal light change only. Such an arrangement is disclosed in the aforementioned MacDonald U.S. Pat. No. 3,704,396. Other prior art systems provide modulation of the light at a predetermined frequency and utilize a tuned receiver circuit to discriminate between the ambient and signal light changes. This kind of system is disclosed in the Bagno U.S. Pat. No. 3,370,284.
The need for self-checking provisions in presence detection systems has been alluded to above. The prior art patents relating to presence detection systems have disclosed some degree of self-checking. For example, it is known to provide self-checking by subjecting the presence detection system to a simulated occurrence of object presence to determine whether the system responds correctly. A system of this type is disclosed in the Engh U.S. Pat. No. 3,543,260. Failsafe operation, as distinguished from self-checking, has also been provided in presence detection systems, such as that shown in the Woodward U.S. Pat. No. 3,242,341.